The present invention relates to an apparatus for producing double-walled tubes of plastic material having an outer wall provided with annular or thread-like helical corrugations and an inner wall which has at least in longitudinal direction and preferably also in circumferential direction a smooth inner surface. The apparatus includes an extrusion head arranged coaxially within tubular mold means formed by a plurality of mold sections traveling along endless paths and abutting against portions of the paths against each other to form a tubular mold having an inner corrugated surface. The extrusion head comprises two coaxially arranged annular extrusion nozzles having outlet ends axially spaced from each other through which plastic material is extruded. The extrusion head further includes a passage for gas under pressure having a discharge end between the outlet ends of the nozzles for pressing the tubular plastic material emanating from the first extrusion nozzle against the inner corrugated surface of the tubular mold to thus form the outer corrugated wall of the double-walled tube, whereas means are provided downstream of the second extrusion nozzle for pressing the plastic material emanating therefrom and forming the inner wall of the tube against the inner crests of the corrugated outer tube wall. In known apparatus of this kind (Pat. No. 3,280,430), the two extrusion nozzles have radially outwardly directed outlet ends and are connected to each other by axial channels, so that the two extrusion nozzles are supplied with plastic material from the same supply channel. The outlet ends of the two extrusion nozzles are axially spaced from each other for a distance which is substantially equal twice the pitch of the corrugations of the outer tubular wall. The discharge end of a passage for gas under pressure is located between the outlet ends of the annular extrusion nozzles to form an overpressure downstream of the first extrusion nozzle to thereby press the plastic material emanating from the first extrusion nozzle against the corrugated inner surface of the tubular mold. A piston having an outer diameter substantially equal to the inner diameter of the inner tubular wall is arranged downstream of the second extrusion nozzle for pressing the plastic material emanating in tubular form from the second extrusion nozzle against the inner crests of the corrugated outer tubular wall.
Due to the mechanical pressing of the tubular plastic material forming the inner wall of the double-walled tube onto the inner crests of the corrugated outer tube, the danger exists that the material of the inner wall will be partly displaced, especially if the aforementioned piston is closely dimensioned in such a manner to assure a proper connection of the inner tubular wall with the inner crests of the outer corrugated tubular wall. If the inner tubular wall is not sufficiently pressed against the corrugated outer tubular wall, the likelihood exists that the inner tubular wall becomes subsequently disconnected from the outer tubular wall and will collapse. This danger is further increased due to the necessary overpressure for pressing the tubular plastic material emanating from the first extrusion nozzle against the corrugations of the tubular mold, since the gas under pressure will be enclosed between the corrugations of the outer tubular wall and the inner tubular wall which closes the corrugations so that the enclosed gas under pressure will tend to move the inner tubular wall radially inwardly of the currugated outer wall. Furthermore, the overpressure between the extrusion nozzles will detrimentally effect a proper adherence of the tubular wall of plastic material emanating from the second extrusion nozzle onto the outer corrugated tubular wall which, in turn, will require increase of the pressure produced by the piston and reduction of the manufacturing tolerances for the piston.
The difficulties resulting from the aforementioned overpressure can be partly avoided by exhausting the air in the corrugation of the tubular mold from the outer surface of the tube of plastic material, as for instance shown for a double-wall corrugated tube in the German Auslegeschrift No. 1,203,940. However, since the mold is necessarily composed of mold sections which only in the region of the outlet ends of the extrusion nozzles abut against each other, such an exhaustion of air must necessarily be carried out through the mold sections, which leads to a very complicated construction.